The Cu and Ba backgrounds potentially could reduce the specificity and thus result in an even more complicated situation than in the elemental spectra. This is where the mass filtering action of the quadrupole mass filter can be utilised. The quadrupole is then set with a mass window function of 16 amu centered around ⁴⁸Ti. This means that Cu ions and Ba ions are discriminated by the mass filter action of the quadrupole mass filter and thus are no longer present in the ScO and TiO spectrum which is now interference free. The resulting mass spectrum is displayed in FIG. 7. Cu ions and Ba ions are removed from the mass spectrum because these ions are discriminated by the first quadrupole mass filter.

Therefore, it can be seen that the method of the invention in one embodiment can comprise: operating the mass filter to mass select the ion beam to transmit only ions within a pre-determined mass range and providing the collision cell with a reaction gas to react with at least one ion of interest in the mass selected ion beam, preferably an elemental ion of interest, thereby producing a mass-shifted ion of interest that lies outside the pre-determined mass range selected by the mass filter. Preferably, the width of the pre-determined mass range is not greater than the mass of the reaction gas. For example, where the reaction gas is oxygen the width of the pre-determined mass range can be 16 amu or less.